Fix a memory leak in uxproxy.c.

We set up a pair of bufchains for the standard input and output
exchanged with the proxy process, but forgot to clear them when the
Local_Proxy_Socket is cleaned up.

(cherry picked from commit bb66e9870e1d297de502767031563b8f2334cb1c)

unix/uxproxy.c

@@ -104,6 +104,8 @@ static void sk_localproxy_close (Socket s)
     uxsel_del(ps->from_cmd);
     close(ps->from_cmd);
 
+    bufchain_clear(&ps->pending_input_data);
+    bufchain_clear(&ps->pending_output_data);
     sfree(ps);
 }